1. Field of the Invention
The present invention relates to a carbon material suitable for magnetic disc substrates used for magnetic discs for high density recording, molds for optical lens, or artificial heart valves, and a process of the production of the same.
2. Description of the Prior Art
With rapid progress of magnetic disc apparatuses, and with high density recording by magnetic discs used as magnetic recording mediums, improvements of properties of magnetic disc substrates are recently demanded as shown below under (1) to (5):
(1) First, with respect to the surface properties of the substrates, in order to make possible high density recording by magnetic discs, the surface accuracy should be excellent, and surface defects should be less.
(2) In order to make the followability of a magnetic head favorable, undulations of fine pitches that deteriorate the surface smoothness and the surface evenness of the magnetic recording disc substrate should be small, and the surface configuration must be free from fine projections.
(3) Since the substrate carries a magnetic medium, the chemical properties should be such that the surface treatment can be done favorably, and the substrate should be non-magnetic.
(4) The substrate should be excellent in corrosion resistance, and weather resistance, and high in strength and hardness.
(5) The substrate should have good floating properties, and should be lightweight.
Under these circumstances, instead of conventional aluminum alloy magnetic disc substrates, a high density recording magnetic disc substrate of a ceramic coated with a glass, and a high density recording magnetic disc substrate consisting of a glass plate have been developed recently. These substrates are excellent in heat resistance and corrosion resistance, and since an excellent surface accuracy can be obtained by abrading the surface, high density recording becomes possible.
However, these materials have a defect that these materials are liable to become brittle to fracture. Therefore, the materials are liable to be broken, for example, by the rotation, impact, marring, and heat shock, and thus the reliability is low.
It is conceivable to use means of increasing the braking toughness by forming a stabilizing layer at the crystal boundary, but brittle fracture cannot be prevent completely.
Since ceramic materials have higher specific gravities, in comparison to aluminum alloy substrates, they will impose a large load on a disc drive system, which makes miniaturization of the drive difficult.
In contrast, the specific gravity of carbon materials is as low as 1.5 to 2.0, and since the thermal coefficient of expansion thereof is small, the heat stability is excellent. Therefore, it is expected carbon materials are practically used, in place of the above-mentioned aluminum alloy or ceramic materials, for high density recording magnetic disc substrates.
Further, of the carbon materials, amorphous carbon has such properties that it is relatively dense, and hardly allows gases to permeate. Amorphous carbon material is conventionally produced by molding, drying, and setting a thermosetting resin, and carbonizing it at high temperature.
However, although amorphous carbon can provide a surface accuracy locally excellent by abrading the surface, in practice it is difficult to prevent the surface being formed with micro pores in the steps of the production.
This is because that although conventional amorphous carbon material has a spherical crystal structure, the void diameter in the crystallite is as high as 100 .ANG., and therefore when the surface is abraded, recesses having a diameter of 100 .ANG. or over appear, and the surface after the abrasion becomes rough. Thus, when the surface accuracy is poor, improvement of the recording density of the magnetic disc cannot be expected.
Further, the reason why the surface accuracy of the conventional amorphous carbon is poor as mentioned above is that a lot of closed pores are formed in the process of the production. That is, after a thermosetting resin is molded, in the course of drying, setting and carbonizing it at high temperature it is inevitable to avoid the occurrence of closed pores due to dissipated moisture and volatile components and air, resulting in the formation of fine recesses in the surface at the time when the surface is abraded.
Therefore, in order to reduce the occurrence of the closed pores, the following methods are suggested:
(1) After a thermosetting resin that is a raw material is molded, it is set by heating at a heating rate of 1.degree. C./hour or below, followed by carbonization.
(2) Low-boiling point materials that are by-products at the time of the setting are dispersed and dissolved perfectly in the matrix resin, and the resin is set with that dispersed state kept (Japanese Laid-Open Patent Application No. 171208/1985).
(3) After a thermosetting resin compound is heat-treated at 300.degree. to 750.degree. C., it is heat-treated under such conditions that the pressure is 1,000 atmospheres or over, and the temperature is 800.degree. C. (Japanese Laid-Open Patent Application No. 36011/1987).
However, in the method (1), the period required for setting and firing that is 2 to 3 months is too long. Therefore, the production efficiency is quite low, the cost is high and it is difficult to make the method industrially and practically possible.
The method (2) has also a defect that since the adjustment, for example, the deaeration under reduced pressure, of the thermosetting resin raw material is complicated, the treatment time is long, and in addition the generated voids cannot be caused to disappear.
Thus, the yield is inevitably low, therefore in either methods, although the amorphous carbon can have fundamentally excellent properties, the production cost is high, and the amorphous carbon has not been industrially employed as general material.
Further, in the method (3), after preliminary heat-treatment at 300.degree. to 750.degree. C., heating is carried out under highly elevated pressure. However, when the preliminary heat-treatment temperature is at most 750.degree. C., large amounts of H, N, O, etc. remain in the material, and gases of H.sub.2, N.sub.2, O.sub.2, etc. are generated during the subsequent step of heating treatment under highly elevated pressure. In the industrial production, in order to lower the cost, if a batch process is used, it is required to make the apparatus large-sized to increase the number of treated products. However, if such a material is treated in a large amount, large amounts of gases of H.sub.2, O.sub.2, N.sub.2, etc. are generated, causing problems including a risk of explosion of the generated gases, the impossible refuse of the pressurizing gas medium, and corrosion of the involved apparatus with the generated gases.
Furthermore, if thermosetting resins are heat-treated, since large amounts of H.sub.2 O, CO, and CH.sub.4 are generated at 400.degree. to 800.degree. C., pores of several millimicrons are generated in a large amount. These pores almost disappear, when the thermosetting resin is heated simply to 800.degree. C. or over, due to the shrinkage of the thermosetting resin resulted from the generated of H.sub.2 (reference: Carbon. 7 (1969), pages 643 to 648). That is, these pores can be caused to disappear easily even if a pressure is not applied. However, a problem is pores that are made of air or the like which is included when the thermosetting resin is molded, and that are not caused to disappear even when it is heated in the above temperature range.